Fuel cells have long been known as electrochemical devices suitable for permitting the direct conversion of free energy from chemical reactions into electrical energy. As such, the fuel cells are not affected by the typical limitations of the Carnot cycle, their energy conversion efficiency is intrinsically higher than that of systems based on fuel combustion, such as motors of the Otto and Diesel type and generally all the burners used for example in civil and industrial heating systems. Further, the operation of fuel cells is characterised by the absence of noxious emissions, such as carbon oxide, aromatic polynuclear hydrocarbons, nitrogen oxide and powders which conversely are a constant of the internal combustion systems. These two positive characteristics and the problem, now universally recognised, of the progressive decay of the environment quality primarily in large urban agglomerates have recently increased the interest for fuel cells for which today new advanced construction materials are available for the internal critical parts as well as new production methods with a sound possibility of mass production under limited costs.
Among the various known types, membrane fuel cells seem to be quite close to the commercial application stage. These cells are essentially made of two porous electrodes provided with suitable catalysts and comprise an interposed thin proton conductive membrane. This assembly is included between two conductive plates having the multiple function of feeding the reactants to the two electrodes, preventing release of the reactants to the external environment and recovering the electrical energy produced by the chemical reactions promoted by the catalysts. A multiplicity of cells is assembled to form a real battery, currently known as “stack”, with the aim of obtaining the high current densities required by conventional applications. For the proper operation of this type of stack, the proton conducting membrane must exhibit a high conductivity in order to minimise the electric energy losses due to the ohmic drops. Currently available membranes are based on sulphonated polymers, and especially perfluorinated polymers, and must be maintained in a highly hydrated condition. This goal is achieved by limiting the operating temperature to 60-90° C. and feeding the above mentioned gases only after suitable humidification.
The reactants which permit the operation of membrane stacks consist of gas containing oxygen, advantageously air, fed to positive polarity electrodes, the cathodes, and of gas containing hydrogen fed to negative polarity electrodes, the anodes. As concerns air, while obviously no supply limitations are faced, the only problems are connected to the need for abating the powders and to the pressure level which, in particular for automotive applications is preferably higher than the atmospheric pressure. As concerns hydrogen, it may be supplied as pure gas under pressure in bottles or as low temperature liquid in particular containers with a highly efficient thermal insulation. However, the availability of pure hydrogen is not immediate today and certainly represents a decisive obstacle to the fuel cells diffusion at least for mass applications, such as stationary applications for the generation of electric energy and heat in civil and industrial buildings and for automotive applications. These applications attain the largest industrial interest, a fact that justifies the paramount financial effort required for the development and for the subsequent demonstration phase. For this reason the technology is today oriented towards the on-site production of hydrogen by means of the known endothermic or autothermic reforming processes from hydrocarbons such as methane, LPG, gasoline, and alcohol such methanol or ethanol. Therefore the fuel cell system herein considered consists of the stack, the reforming reactors, auxiliary components such as the fans/compressors, circulation pumps, heat exchangers, valves and the control electronics.
The steam reforming reaction between hydrocarbon or alcohol and water in a first high temperature step produces a mixture of hydrogen, carbon monoxide, carbon dioxide, water and in some cases nitrogen, which are subsequently converted at a lower temperature into hydrogen, carbon dioxide, residual water and in some cases nitrogen (operation commonly known as “CO-shift”). This final mixture still contains small quantities of carbon monoxide, generally comprised between 0.5 and 1% depending on the conditions adopted during the CO-shift phase.
While carbon dioxide has no negative effect, conversely the presence of carbon monoxide (CO) represents a serious problem for the regular operation of the catalysts incorporated into the anodes of the stack, as CO is capable of adsorbing onto the active sites which are thus made no more available for the necessary reaction with hydrogen. At the operating temperature of prior art membrane fuel cells, typically 60-90° C., adsorption is particularly intense and is eliminated only when the content of CO in the gas coming from the reforming unit is extremely reduced, in particularly below 10 parts per million (ppm). This result is achieved immediately downstream the CO shit unit in an additional selective oxidation reactor provided with very specialised catalysts: the gas containing hydrogen, carbon dioxide, water, in some cases nitrogen, and about 0.5-1% of CO, after the addition of suitably tailored quantities of air, is injected inside the reactor where the oxygen contained in the air oxidises the CO to carbon dioxide. This device is negatively affected by a series of inconveniences, and precisely: loss of hydrogen, which, even if only partially, combines with oxygen to form water, with consequent temperature increases dangerous for the selective oxidation reactor, dilution with nitrogen contained in the air, criticality of the regulation, casting doubts on the reliability of operation with time, possible loss of control in the case of heavy peaks of CO contents which may reach the stack, with additional costs for the system.
An alternative solution for operating membrane fuel cells also with reforming gas containing CO concentrations exceedingly higher than the critical limit of 10 ppm is described by R. A. Lemons in the Journal of Power Sources, 29 (1990), page 251: the reforming gas with the addition of suitable quantities of air immediately upstream the stack exhibit performances very close to those obtained with a similar gas totally free from CO.
However, also the method disclosed by Lemons requires a very accurate regulation system which may prove to be not completely reliable with time and which certainly increases the costs of the overall system. A possible failure of the air injection device may entail dramatic consequences such as the possibility of causing a temperature increase at the anodes with the possible irreversible deactivation of the catalyst and in the worst case the formation of explosive hydrogen-oxygen mixtures inside the stacks. An alternative embodiment of this concept, which permits to avoid the air flow regulation devices, is described in DE 19646354. The oxygen necessary for the oxidation of carbon monoxide is produced in a water electrolysis cell and is added to the gas containing hydrogen and carbon monoxide: in this case the percentage of oxygen is maintained constant even when the output of electric energy produced by the stack varies, by suitably adjusting the current fed to the water electrolysis cell. It is therefore necessary to resort to a suitable regulation device which again brings forth costs and problems similar to the aforementioned problems connected to the regulation of the injected air flow. DE 19710819 describes a method for reactivating a fuel cell affected by a performance failure due to the carbon monoxide present in the gas containing hydrogen, which method is based on the periodical short-circuiting of the cell during operation. It may be assumed that during short-circuiting the anode undergoes an increase of its electrochemical potential up to a level wherein the water present at the anode-membrane interface is converted into OH. and O: radicals adsorbed onto the surface of the catalyst particles. These radicals are able to transform the carbon-monoxide blocking the catalytic sites into inert carbon dioxide which is desorbed. At the end of the short-circuiting the catalyst may thus operate regularly; however, in the absence of a continuous protection, the performances are spoiled again. This involves the need for periodically repeating the short-circuiting step. It is obvious that this method applied to a stack operating in a commercial system would cause periodical variations of the current output, a fact unacceptable for the user.
To overcome the problems connected with the above illustrated methods, a large part of the efforts for developing the membrane fuel cell technology have been focused on the optimisation of anodic catalysts intrinsically resistant to the presence of CO concentration of at least 100 ppm in the gas coming from the reforming and CO shift units. These catalysts are made of platinum mixed with other noble or non noble metals, such as for example ruthenium and molybdenum, the latter described in U.S. Pat. No. 6,165,636. The principle on which these formulations are based is presumably twofold: weakening the CO adsorption energy and strengthening the hydrogen absorption energy, thanks to the electronic structures of these alloys, and enhancing the formation of absorbed oxidising species, such as OH., on the catalytic sites supplied by the added metal, as in the case of ruthenium and molybdenum. These catalysts appear to be rather delicate: for example molybdenum may undergo an irreversible oxidation which destroys all its resistance properties under an excessive current output or when a problem in maintaining the reforming gas flow rates is experienced. In the most favourable hypothesis, the data so far available indicate that the approach based on catalysts resistant to poisoning will permit to use gases containing up to about 100 ppm of CO: in this case the reforming and CO shift units will have to be equipped with the selective oxidation reactor, even if the required guaranties as concerns the purity of the produced gas may be less severe, with simplification of the design. However the possibility of damages in case of strong anomalies affecting the composition during shut-downs or performance failures is not yet eliminated, and its elimination requires for additional controls onto the stack and/or the reforming and CO shift reactors.
A further approach to the solution of the problem of poisoning due to carbon monoxide foresees the operation of membrane fuel cells stacks at substantially higher temperatures than the previously specified values of 60-90° C. For example at 150-180° C. the adsorption of carbon monoxide is practically negligible and reforming gas containing even 0.5-1% carbon monoxide may be directly used without any pre-treatment. Membranes capable of operating satisfactorily under these conditions are presently under study and, taking into account the problems to be solved, such as mechanical stability and acceptable electrical conductivity with low hydration levels, it will take a long time for the first commercial applications.
The critical analysis of the above illustrated prior art shows that today there is a strong need for devices and methods for feeding the stack with gas containing high concentrations of CO, by far higher than 100 ppm, in a simple, intrinsically reliable and reasonably not expensive way.